Tool fastening device

In a quick change device for a hollow spindle of a machine tool, in particular a milling machine arranged for the quick releasing and clamping of a conical tool holder, said tool holder having a shaft with a ring groove at the small end of the shaft, a coupling casing, said coupling casing being attached to the spindle and having a flange, a cylindrical drawbar axially shiftable within the casing, clamping spring means about the casing pressing on one hand against said flange, a tool holder drawbar connection including radially movable clamping jaws attached to said longitudinally shiftable cylindrical drawbar, an expansion rod connected to said coupling casing and being located within the drawbar for actuating the said jaws of the drawbar as the drawbar pulls the holder into the spindle under the influence of clamping springs, the clamping jaws being detachable upon compression of the clamping springs through an external means, and a releasable coupling between said drawbar and said clamping springs comprising a plurality of bores in said coupling casing and a group of radially movable balls retained in said bores, the drawbar rod being provided with a groove, a ring with a hollow cone shaped surface surrounding the drawbar, said spring means acting on the other hand against the ring, the balls being radially pressed to the inside of the drawbar groove by said ring on which the clamping springs are acting.